The invention relates to an electrode head, for instance for electrochemical potential measurement electrodes. The head comprises an electrode cap for connection with an electrode rod generally consisting of glass or plastics. Contacts are located in the electrode cap for connection with discharge connections from the electrode rod and a coaxial cable having an external conductor which serves, in general, as a screen and which is insulated from one or more internal conductors in the cable. The invention also relates to a process for the production of such an electrode head.
Measuring electrodes usually have a high internal resistance (.about.10 M Ohms). The connecting cable is consequently formed as a coaxial cable to ensure electrostatic screening. The core of the cable is connected with the actual measuring system. The electrostatic screening concentrically surrounding the cable core usually takes the form of a copper mesh and is connected either with the electrostatic screening or shielding within the electrode concentrically around the measuring system or, in the case of single rod measuring circuits, together with the reference system, simultaneously serving as a screen.
In electrode heads of such electrodes, the internal conductor of the potential transmitting element is generally connected to the electrode cable. The electrostatically screened potential is transmitted to the measuring instrument by means of the electrode cable. In the case of single rod measuring circuits, the reference electrode signal is also transmitted by way of the screening mesh.
A number of such electrode head designs is already known.
In one such design, an outer electrode head is in the form of a metal or plastics sleeve to which a cable grommet or a water-tight bush is fitted. Assemblies are also known in which the electrodes are fitted by screwing them into the head. Other designs favor a cylindrical rubber cap through which the cable is introduced from the top and which is secured to the glass electrode body. The cap in such designs is connected to the glass electrode body either by bonding with epoxy resin, silicone resin or other adhesive or cement or by simple press-fitting. The cable at such an electrode is connected with the internal conductor of the electrode within the so-called electrode head. This connection or link cable is connected with the internal conductor by soldering or welding, or by press contacts. In other designs the cable core extends further into the electrode and contact is effected with a plug contact or by means of metal wool within the electrode. The reference electrode connection is then connected by pressing or by soldering with the screening, transmitting the reference electrode signal to the measuring instrument.
All designs known hitherto have one or more of the following disadvantages:
The screening, i.e. in a single rod measuring circuit of the reference electrode connection, is in the form of an external jacket which is not insulated from the environment. In the event of moisture or manual contact with the head of the electrode, the reference system is short-circuited by way of the resultant shunt resistor or there is a double grounding;
The electrode head is secured mechanically by a screw arrangement, but with sufficient moisture-protection, so that the insulation resistance required for measurement purposes is not maintained in the long term between the cable core and the screening of the coaxial cable;
The electrode head has no shielding protection at all in the case where a metal sleeve is used as the electrode head, and the metal sleeve is not connected to the screen system. Such electrode designs are hand-sensitive as a result of electrostatic discharges;
The cable core and screening are connected in each case by soldering to the internal contacts. The soldering is subject to the hazard of cold joints and is also highly costly.